It is often necessary to provide for information (data) communication amongst any pair of a plurality of transmit-receive stations. FIG. 1 illustrates, in simplified block diagram form, a communication system 10 including a plurality of transmit-receive (transducing) stations (ST) 12.sub.1, 12.sub.2, 12.sub.3 . . . 12.sub.K, and another plurality of stations 14.sub.1, 14.sub.2, 14.sub.3 . . . 14.sub.L, interconnected by (N+C)-bit data paths designated 13.sub.1, 13.sub.2 . . . 13.sub.K, 15.sub.1, 15.sub.2 . . . 15.sub.L and by a crossbar switch 16. Crossbar switch 16 includes a set of K+L data ports, including a plurality K of ports, each of which is coupled by a data path 13.sub.1, 13.sub.2, . . . , 13.sub.K to one of stations 12.sub.1, 12.sub.2, . . . 12.sub.K, respectively, and also including a further plurality L of ports, each one of which is coupled by a data path 15.sub.1, 15.sub.2, . . . , 15.sub.L to one of stations 14.sub.1, 14.sub.2, . . . 14.sub.L, respectively. As illustrated in FIG. 1, each data path 13, 15 includes a number N+C of parallel data paths, N of which carry digital signal bits of significance ranging from an least-significant bit (LSB) to a most-significant bit (MSB); one of the N data paths associated with each station carries the LSB, another of the N paths carries the MSB, and each of the other N bit paths carries bits of a particular significance lying between the LSB and MSB. The C bit paths are used for error Coding, such as for error detection and correction (EDAC) or parity coding. For example, station 12.sub.1 communicates through data path 13.sub.1, by means of an (N+C)-bit digital signal, including an LSB and an MSB, each of which is carried by a separate bit path (ordinarily one conductor wire) of the N portion of data path 13.sub.1, and carries error coding bits in the C portion of data path 13.sub.1. Similarly, station 14.sub.1 communicates by means of an N-bit digital signal and C error coding bits through data path 15.sub.1, which has (N+C) bit paths. It should be noted that some or many prior art communications systems may dispense with error coding, whereupon C=0, and communication system 10 of FIG. 1 becomes an N-bit system.
Crossbar switch 16 as illustrated in FIG. 1 includes a plurality 1, 2, 3 . . . K "upper" ports connected to stations 12.sub.1, 12.sub.2, 12.sub.3 . . . 12.sub.K, and includes a further plurality of "lower" ports 1, 2, 3 . . . L, which are connected by way of data paths 15 to stations 14.sub.1, 14.sub.2, 14.sub.3 . . . 14.sub.L. The separate designations should not be construed to mean that there is any difference among the ports. Thus, there is no necessary difference among any of the stations 12 and any of the stations 14, and they could all have easily been designated by a single reference numeral, such as 12, with a different set of subscripts. Similarly, there is no difference among any of the information ports of crossbar switch 16.
In some instances, it is customary to provide redundancy to avoid complete system failure in the event of failure of a single component or device. For example, in the arrangement of FIG. 1, if crossbar switch 16 became inoperative, as might occur in the absence of error coding if even one bit path of the N bit paths became open or shorted to ground in the switch or its interconnecting paths, or in the presence of error coding if a number of bit paths of the N bit paths, exceeding the number for which the error coding corrects, became inoperative, the entire communication system might become nonfunctional.
FIG. 2 illustrates an arrangement generally similar to FIG. 1, in which two crossbar switches, designated 16a and 16b, are paralleled. In FIG. 2, elements corresponding to those of FIG. 1 are designated by like reference numerals. In FIG. 2, each output port of crossbar switch 16a is paralleled with the corresponding output port of the redundant crossbar switch 16b. For example, data port 1 of the lower set of L ports of crossbar switch 16a is connected in parallel with data port 1 of the lower set of L ports of crossbar switch 16b, as exemplified by data path 221.sub.1.sup.a which connects data port 1 of the upper set of ports of switch 16a to data path 15.sub.1, and a similar data path 22.sub.1.sup.b connecting data port 1 of the lower set of ports of switch 16b to data path 15.sub.1. In this context, the term "parallel" means that each bit path of data path 22.sub.1.sup.a is connected to the corresponding bit path of data path 22.sub. 1.sup.b. The arrangement illustrated in FIG. 2 provides redundancy of the crossbar switch, so that a failure of the switch, or of a portion thereof, may be overcome by use of the alternate or redundant crossbar switch.
The arrangement of FIG. 2 does, however, have some limitations in the level of achievable redundancy. The arrangement of FIG. 2 provides for redundancy of data paths such as data path 22.sub.1.sup.a and 22.sub.1.sup.b, in that an open-circuit failure in one of the data paths can be overcome by switching to the redundant crossbar switch, which also switches the data path 22.sub.1. However, a short-circuit or inadvertent interconnection of one bit path of a data path to another bit path of the same data path, or to ground, cannot be corrected, as a result of the parallel connections of data path 22.sub.1.sup.a to 22.sub.1.sup.b. Also, the arrangement of FIG. 2 by implication requires some means for detecting the existence of a failure associated with the crossbar switch. In the simplest situation, this might involve a human operator who observes the system and who, in response to an overt system problem such as a broken or failed bit or data path (wire or fiber-optic cable), or in response to inappropriate system behavior, controls the system so as to operate with the alternate crossbar switch. Faster and more reliable operation might be achieved with an automatic error detection system, for detecting the presence of errors by comparison of parity bits or the like. If such an error detection system were associated with one of switches 16a or 16b of FIG. 2, a single failure in the error detection system itself might result in an inability to switch in the presence of a failure in the data paths. The provision of an additional crossbar switch may not be the most cost-effective way to provide fault tolerance in such a system, and may also adversely affect system performance due to additional signal loading attributable to the parallel connections.
An improved multiple station communication system for parallel digital signals is desired.